The present invention relates to a superconducting quantum interference device (hereinafter referred to as a SQUID). More particularly, the present invention relates to a SQUID which includes a superconductive loop, one or two Josephson junctions (hereinafter referred to as JJs) formed at predetermined positions on the superconductive loop, an input coil for efficiently guiding magnetic flux to the superconductive loop on which one or two JJs are formed (hereinafter referred to as SQUID loop), and a modulation coil for supplying magnetic flux to the superconductive loop for compensating for variations in the external magnetic flux, as a principle arrangement, and which SQUID is integrated in its entirety on a substrate.
It is known that a SQUID is capable of detecting magnetic flux with extremely high sensitivity. With attention to this characteristic, a SQUID is applied to various apparatus which are used in various technical fields. A SQUID is classified as an rf-SQUID if it has only one JJ and as a dc-SQUID if it has two JJs. The rf-SQUID was generally used in the past years, while the dc-SQUID is being widely used in recent years because two JJs having similar characteristics can be obtained due to improvements in thin film manufacturing engineering in recent years.
FIG. 7 is an electrical diagram for explaining the principle of a dc-SQUID magnetic flux meter.
The dc-SQUID includes a superconductive loop 51 and two JJs 52 which are provided at predetermined positions on the superconductive loop 51. A bias current is supplied to the opposite positions on the superconductive loop 51 with respect to the JJs 52. An input coil 53, which is interconnected with a pickup coil (not shown) for detecting the magnetic flux of an object under measurement, is provided at a closed position on the superconductive loop 51. A modulation coil 54 which is used for performing magnetic flux locked loop operation, is further provided.
FIG. 8(A) is a plan view showing an arrangement of a conventional SQUID, while FIG. 8(B) is a cross sectional view thereof.
An input coil 53, a superconductive loop 51, a modulation coil 54 and JJs 52 are formed in a lamination layer manner. The SQUID also includes wirings 55 and layer insulation layers 56.
When the SQUID having the arrangement above-mentioned is employed, the input coil 53 can be positioned at a closed position with respect to the superconductive loop 51 so as to improve magnetic flux guiding efficiency to the superconductive loop 51 by the input coil 53.
The SQUID having the arrangement above-mentioned is designed whereby its input coil has minimum line width so as to decrease the outside dimension of a chip. The input coil of the SQUID is formed below a SQUID loop which includes the superconductive loop and the JJs. With this SQUID it is almost impossible to form taps, accordingly. The SQUID is required to have an inductance L of the input coil which has a spiral shape, set to an optimum value in correspondence with the design of an externally attached pickup coil for which environmental conditions should be taken into consideration, so as to increase the magnetic flux guiding efficiency to a maximum value. SQUIDs having corresponding input coils with different numbers of turns are accordingly manufactured in correspondence with pickup coils.
A disadvantage arises in that manhours for designing and for manufacturing increase due to the necessity of preparing mask patterns corresponding to the numbers of turns of input coils. Disadvantages also arise in that the cost of the SQUID is increased, manufacturing efficiency is lowered, reliability of the SQUID is lowered and so on, due to the sizes of chips being different from one another in correspondence with the numbers of turns of corresponding input coils, and the necessity of preparing packages in correspondence with the chips having different sizes.
A SQUID is easily influenced by electromagnetic noises, therefore the SQUID is housed in a superconductive shield using a niobium tube so as to improve magnetic field measurement accuracy when a feeble magnetic field such as the magnetic field of a living organism is to be measured. The superconductive shield using a niobium tube has great heat capacity. Disadvantages arise in that liquid helium loss becomes great when a refrigerating system using liquid helium is employed, and in that it requires a long time period for cooling down a SQUID when the refrigerating system using a refrigerator is employed. It is proposed that a SQUID having a spectacle type arrangement be employed so as to cancel the influence of external magnetic fields and to omit a superconductive shield, by taking the disadvantage above-mentioned into consideration. A disadvantage arises in that trapping of magnetic flux easily occurs because a SQUID is exposed to external magnetic flux when the SQUID is cooled down.
A SQUID needs layer insulation layers when each layer is formed in a lamination manner. The SQUID should be prevented from having its characteristics lowered when each layer insulation layer is formed. Thereby, a film forming process with high temperature cannot be employed. Disadvantages arise in that good layer insulation layers cannot be obtained, in that fine processing is difficult to perform because good flattening of the SQUID cannot be performed when the number of layers increases, and in that the yield of SQUIDs is lowered because disconnection and the like occurs when fine processing is forcibly employed.